Facies Architecture and Cyclicity of an Upper Carboniferous Carbonate Ramp Developed in a Variscan Piggy‐Back Basin (Cantabrian Mountains, Northwest Spain)

Abstract

The latest Moscovian (late Myachkovsky) to Gzhelian succession in the northern sector of the Picos de Europa Province (Cantabrian Mountains, northwest Spain) was deposited in a rapidly subsiding piggy-back basin. The succession has been subdivided into 11 mappable depositional sequences (3rd-4th order), which have in turn been grouped into two sequence sets, and they are formed of several higher order cycles (4th-5th order). This work focuses on the study of a carbonate ramp system (Puentellés Formation, northern proximal deposits of Sequences 8-10), where the cyclicity is best developed. Each of the three depositional sequences forming the Puentellés Formation comprises a fining upward lower part, which consists of several higher order (metric to decimetric) cycles with similar internal organization, and an upper part mainly arranged in metric to decametric shallowing upward parasequences. The results of this study suggest that tectonic activity controlled the configuration and long-term development of the latest Moscovian (late Myachkovsky) to Gzhelian basins, being responsible for the large-scale stratigraphical architecture (sequence sets). Eustasy, on the other hand, was the driving mechanism for the higher frequency architecture (3rd-5th-order cyclicity).The subsidence curves are similar for both sequence sets, showing a maximum subsidence rate in the beginning that gradually decreased, reflecting the tectonic load during two major phases of thrust-sheet emplacement. The unconformity between both sequence sets records an intense phase of erosion linked to uplift of the northern sector of the Picos de Europa Province due to the onset of the emplacement of the Picos de Europa thrust sheets. The tectonic deformation was also responsible for the angular unconformities and the syntectonic unconformities that bound the depositional sequences, but the unconformities in themselves were a consequence of eustatic sea-level falls. The lower part of each sequence in the Puentellés Formation records late lowstand clastic sedimentation in flood-dominated deltas and fan-deltas, which co-existed with a reduced carbonate production in a narrow shallow-water ramp. The minor unconformities that bound the 4th-5th-order cycles are only present in the lower part of the 3rd-order sequences, and are interpreted to have developed when minor tectonic uplift was enhanced during lowstand stages. The upper part, mainly composed of autochthonous carbonates (including microbial and algal boundstones, among others), represents the abandonment of the previous clastic systems and the encroachment and aggradation of the carbonate ramps during rising and highstand stages.The development of the Puentellés carbonate ramp was exceptional in that, normally, terrigenous influx typical of active tectonic regimes adversely affects carbonate production, preventing the development of carbonate depositional systems. In this case, the clastic supply was mainly calcareous, due to the composition of the source area, and as a consequence, almost no terrige...

Emplacement of the Cuera and Picos de Europa imbricate system at the core of the Iberian-Armorican arc (Cantabrian zone, north Spain): New precisions concerning the timing of arc closure

“…9). This unconformity postdates older thrusts that were active during the deposition of the lower sequence set (Merino Tomé, 2004;Merino-Tomé et al, 2007).…”

“…In the northern domain, mostly calcareous sediments of the Puentellés Formation accumulated, and they are interpreted to represent a shallow-water thrust-top carbonate ramp (Merino Tomé, 2004;Merino-Tomé et al, 2007). The sequence boundaries coincide with angular unconformities formed during subaerial exposure and karstifi cation.…”

“…1A) was emplaced during the last stages of the Variscan orogeny (Merino-Tomé, 2004;Merino-Tomé et al, 2007), most probably coinciding with the closure of the Iberian-Armorican arc (Julivert and Arboleya, 1986;Álvarez-Marrón and Pérez-Estaún, 1988;Pérez-Estaún et al, 1988;Marquínez, 1989;Álvarez-Marrón, 1995;Weil, 2006). The synorogenic successions (late Moscovian-Gzhelian in age, Upper Pennsylvanian), which accumulated in wedge-top basins that developed on top of this imbricate system, provide a valuable record of the evolution of this sector of the Variscan foreland fold-and-thrust belt.…”

New data (smaller foraminifers and correlation with depositional sequences) on the Las Llacerias section (Late Pennsylvanian, Cantabrian Mountains, NW Spain)

“…Merino-Tomé (2004) and Merino-Tomé et al (2006 provided a new sequence-stratigraphic framework for these strata, which were arranged into eleven third to fourth-order depositional sequences (numbered from 1-11). As discussed in Merino-Tomé et al (2007), the Gamonedo-Cabrales third to fourth sequences might be the response to high-amplitude sealevel Xuctuations of global signiWcance which, potentially, could become an important tool for establishing long-distance correlation.…”

“…5;Merino-Tomé et al 2007) and in other Pennsylvanian successions worldwide (Busch and Rollins 1984;Ross 1985, 1988;Fig. 2 Legend for stratigraphic logs (abbreviations: M mudstone, W wackestone, P packstone, G grainstone, R rudstone, B boundstone, Sh shales, Si siltstone, f Wne-grained, m medium-grained, c coarse-grained, vc very coarse-grained, Ss sandstones, C conglomerates) Fig.…”